1. Field of the Invention
The present invention relates to vehicle front end suspension systems and, more particularly, to an air spring based suspension system directed to improving vehicle stability by reducing front end dive during braking and increasing resistance to vehicle roll during cornering.
2. Background
Truck suspension systems must meet each of several not wholly compatible goals which include: absorbing road shock; stabilizing the vehicle; and maintaining proper axle spacing and alignment. These goals must met while supporting the vehicles weight over a wide range of vehicle load conditions and maintaining the driver""s ability to steer the vehicle.
There are four basic categories of suspension systems used on trucks: spring systems, equalizing beam systems; torsion bar systems: and air spring systems. Air spring systems have recently gained in popularity and have been applied to of variety of truck axles, including of particular interest here, the steering axle. Air spring suspensions give excellent load and vibration isolation to the cab by eliminating the interleaf friction found in traditional multiple leaf spring designs. The deflection rate of air springs can be adjusted automatically to compensate for vehicle load changes. As a result, vehicle height does not vary with load or positioning of the load. In addition, an air spring usually has a lower deflection rate than a leaf spring exerting the same force giving the system greater capacity for absorbing shocks for a given displacement between the axle and the frame.
Air springs are also employed to maintain a constant vehicle height despite changes in vehicle loads. As such it may sound odd to refer to a deflection rate for such springs since the deflection rate for a compression spring equates spring deflection with force generated. Air springs, unlike conventional springs, can and are used to generate a varying amount of force while maintaining a fixed height displacement. This is effected by changing air pressure in the air spring in response to changes in vehicle height, either dumping or adding air to the air spring by valves. Conventional springs must of course deflect to generate a balancing counterforce. In effect, as air pressure is changed in an air spring in order to maintain a constant height, the deflection rate of the spring is changing. Thus, air springs may be termed controllable rate springs or controllable deflection springs.
In an air spring based system, air bellows are positioned with respect to an axle and a vehicle frame to support the frame from the axle. The air spring can be used to supplement a leaf spring arrangement by being placed between the leaf spring and the vehicle frame. Commonly though, air spring systems replace the leaf spring. In a typical application of air springs to a steering axle, an ail spring is placed adjacent each wheel over the axle and directly below the side rails of the vehicle frame. The steering axle is than stabilized using trailing connecting rods or arms between the frame rails and the steering axle. Lateral stability for the axle is provided using a lateral track bar. Where a leaf spring provided two frame mounting points fore and aft of the steering axle to aid in stability, the rigid trailing arm has only one mounting point to the frame forward of the axle. Trailing arm systems achieve substantial front end anti-roll stiffness by positioning rigid arms between the frame and the axle, with each arm being pivotally attached to the frame and rigidly attached to the axle. Trailing arm systems have proven simpler than leaf spring designs since the arms can be constructed from a steel box beam.
The trailing arm design used with air springs at the vehicle front end is not without disadvantages. During vehicle braking, the front ends of vehicles tend to dive. In traditional leaf spring suspension designs, where the leaf spring is mounted to the frame at two points, ahead of the solid axle and following the axle, the torque reaction force generated by the brakes on the axle in turn generates a reactive upward force on the frame through the leaf springs aft mounting point and a downward reactive force through the forward mounting point. No net downward force is transmitted from braking. In trailing arm/air spring suspension designs this balance is lost. Trailing arm designs transmit the brake reaction torque to the frame only through the forward trailing arm mount as a downward force and thereby increase dive. Most trailing arm designs are also poor at maintaining axle position laterally, necessitating the use of a lateral track bar to hold axle position.
Arrangements combining the use of air springs and leaf springs, while effective, lack the simplicity of the trailing arm design. Substituting a leading arm for the trailing arm would help counter the dive problem because a leading arm would transfer brake torque to the frame as an upward force. However, steering system arms and draglinks usually include a connecting rod running from a steering gear, which is mounted to the frame at a point forward to the steering axle, back to the steering axle. This rod is thus positioned as a trailing arm, giving the axle and steering arm a pivot point forward of the axle position. Were a leading arm now attached to the steering axle, the steering axle would have an additional pivot point aft of the axle, position. The steering axle cannot rotate on both a trailing pitman arm/draglink pivot point and a leading arm pivot point simultaneously. The result of such a combination is to introduce suspension steering error, compromising driver control of the vehicle.
It is an object of the invention to provide an air spring suspension system with improved vehicle stability characterized by increased resistance to front end dive on braking.
It is a further object of the invention to provide an air spring suspension system having improved resistance to roll.
It is another object of the invention to provide a suspension system which enhances steering axle lateral stability without use of a track bar.
The foregoing objects are achieved as is now described, The invention provides a suspension system for a frame from a steering axle. First and second controllable rate springs are mounted to locally support a frame from the steering axle. A trailing V-link member is mounted to pivot at its apex on a cross-member of the frame positioned forward of the steering axle. The opposite ends of the V-link member are pivotally connected to opposing ends of the steering axle allowing the V-link member to constrain fore to aft movement of the steering axle, lateral displacement of the steering axle and frame roll. A trailing draglink coupled from the frame to a wheel knuckle provides steering control. First and second shackles depend from the frame side rails aft of the steering axle. The first and second leading arms are suspended from the shackle boxes to permit longitudinal translation of the first and second leading arms relative to the frame. The leading arms are rigidly connected to the opposing ends of the steering axle to constrain rotation of the steering axle and the trailing V-link member about the vertex of the V-link member. Lastly, shock absorbers for damping motion between the steering axle and the frame are positioned adjacent the controllable rate springs.
The foregoing objects are achieved as is now described. The invention provides a suspension system for a frame from a steering axle. First and second controllable rate springs are mounted to locally support a frame from the steering axle. A trailing V-link member is mounted to pivot on its vertex on a cross-member of the frame positioned forward of the axle. The opposite ends of the V-link member ale pivotally connected to opposing ends of the steering axle allowing the V-link member to constrain fore to aft movement of the steering axle, lateral displacement of the steering axle and frame roll. A trailing draglink coupled from the frame to a wheel knuckle provides steering, control. First and second shackles depend from the frame side rails aft of the steering axle. The first and second leading arms are suspended from the shackle boxes to permit longitudinal translation of the first and second leading alms relative to the frame. The leading arms are rigidly connected to the opposing ends of the steering axle to constrain rotation of the steering axle and the trailing V-link member about the vertex of the V-link member. Lastly, shock absorbers for damping motion between the steering axle and the frame are positioned adjacent the controllable rate springs. In another embodiment of the invention, the V-link may actually be a curved U-shape and will be engaged to the cross member in two locations with a bushing at each location.
Additional effects, features and advantages will be apparent in the written description that follows.